Purple Marsh Crab (Afrithelphusa monodosa)

Purple Marsh Crab (Afrithelphusa monodosa) downloaded 10/02/2009

Location and Natural Habitat

The Purple Marsh Crab inhabits the year round (not seasonal) marshland in northwestern Guinea's savannah zone. The original habitat is believed to be freshwater wetlands, but this species was discovered in cultivated land near a village, which has been built on what was previously a natural wetland.
The species was first discovered in 1947, but a living specimen was not found until 2005. The Purple Marsh Crab lives in holes dug into dry ground, but water is always found in the bottom of the hole, which leads scientists to believe the species must live near natural springs or at locations with high water tables.
Location of Purple Marsh Crab, downloaded and modified 10/05/2009


The Purple Marsh Crab is one of 5 species of endangered freshwater crab in this region of Africa. Not much is known about this particular species of crab, as only 20 living organisms have ever been found. It can be assumed, due to behavioral similarites, that the Purple Marsh Crab eats algae, bacteria, and dead plant matter like other freshwater crabs do. The species is so rare, very little is known about its reproductive behavior. The maximum size of the species is not known, but they are noted as having very long legs compared to the other freshwater crabs in the region.
The Purple Marsh Crab is an exceptional air breather, as it has well developed pseudolungs and is able to live away from freshwater streams and springs. It digs burrows in agricultural soil and spends the day in the bottom of the cool, damp burrow. At the end of the six-month dry season, the burrow of the A. monodosa still has water in it, despite the fact that the surround surface land is very dry. This leads scientists to believe the crab only survives where there is a high water table or a natural spring supplying fresh water beneath the surface. This enviromental requirement might explain why the living samples of this organism were found in a group of burrows near a farming village. It may not be that the crabs are social or pack animals, but that they have very specific environmental needs that force them to live in close proximity. This may help increase the likelyhood of successful reproduction.
A. monodosa remains in it's burrow during the day and feeds at night. Many burrowing crabs, both diurnal and nocturnal, cover the entrance to their burrow in mud or dirt when the crab is inside, in order to protect it from predators and maintain temperature.
One thing that separates A. Monodosa from the other freshwater crab species in the region is its ability to breath out of water. This has allowed the species to move away from the freshwater springs and springs. However, this has also resulted in loss of habitat as drier regions, where the Purple Marsh Crab lives, are often cultivated as farm land, instead of being left alone.
Land Crab Burrow, downloaded 10/05/2009

Population Status

The Purple Marsh Crab has recently (2008) been downgraded from Critically Endangered to Endangered because the living samples were found in a separate location than the original discovery in 1947. Because of the expanded region, the International Union for Conservation of Nature has increased its population projections for the species, and reduced its dire status. The pressure being placed on this species is due to habitat loss and human encroachment. In addition to the man caused destruction of habitat, increased human populations have also degraded the groundwater supply which is also negatively affecting the habitat of the Purple Marsh Crab.
The Endangered Species Act of 1973 was written to prevent the people and businesses of the United States from driving extinct any species, and this would be prevented at any cost. The Act goes into great specificity about what constitutes a species (versus sub groups of the same species) and the past, current and potential future status of the population. The Act also outlines procedures for identifying what is driving the species extinct and then implements a series of rules that protect the species from future harm, depending on what pressure is killing the organism in the first place. However, the ESA only affects the United States, and has no control or impact on organisms outside of the United States. Therefore the ESA has no bearing on the status of the Purple Marsh Crab.
The present day population of A. monodosa is estimated at less than 2500 adults. This number has been calculated based on very scarce findings, and cannot be relied on for conservation purposes. The population continues to decline as habitat loss and agricultural presure continues. Because of the small amount of information we have about A. monodosa, no predictions have been made about a population recovery or extinction timeline.

Conservation Efforts

As mentioned above, the ESA does not protect species outside of the United States. The poorly developed country of Guinea is not concerned with habitat destruction of what could be considered "inconsequential" species, and currently has few efforts to preserve their populations. The farmers that work the land which is habitat for A. monodosa harvest enough food to support their villages, but not enough that they can change their farming practices enough to prevent destroying the ecosystem for the Purple Marsh Crab. Currenly there are no efforts in place to protect the Purple Marsh Crab in Guinea, and their species may only survive if there are large numbersof them in areas uninhabited by man, or the Guinean government introduces protections for them.

Work Cited

Cumberlidge, N. 2008. Afrithelphusa monodosa. IUCN 2009. IUCN Red List of Threatened Species.
Version 2009.1. Retrieved October 4, 2009, from www.iucnredlist.org

Fiddler Crabs - Bay Field Guide. (n.d.). Retrieved October 4, 2009, from
Chesapeake Bay Program website: http://www.chesapeakebay.net/fiddler_crab.htm

IUCN - Global freshwater crab assessment. (2009, January 19). Retrieved October 4, 2009, from International Union for Conservation of Nature website:

Tony, S. A. (2001). ESA, Endangered Species Act. USA: American Bar Association